Membrane materials used for roof structures of buildings provide a relatively low thermal insulation capacity compared to the classic building materials. Therefore, a large amount of heat penetrates daily through such roof structures into the building especially during the summer months leading to an overheating of the buildings interior. On the other side, the nightly heat loss through such roof constructions, especially during the winter months, is significantly high.
The problem can be solved by applying phase change material to membrane materials used in fabric structures. Phase change material is a highly-productive thermal storage medium which possesses the ability to change its physical state within a certain temperature range. When the melting temperature is obtained during a heating process, the phase change from the solid to the liquid state occurs. During this melting process, the phase change material absorbs and stores a large amount of latent heat. The temperature of the phase change material remains nearly constant during the entire process. In a cooling process of the phase change material, the stored heat is released into the environment in a certain temperature range, and a reverse phase change from the liquid to the solid state takes place. During this crystallization process, the temperature of the phase change material also remains constant. The high heat transfer during the melting process and the crystallization process, both without any temperature change, is responsible for the phase change material's appeal as a source of heat storage.
In order to contrast the amount of latent heat absorbed by a phase change material during the actual phase change with the amount of sensible heat absorbed in an ordinary heating process, the ice-water phase change process will be used. When ice melts, it absorbs an amount of latent heat of about 335 J/g. When the water is further heated, it absorbs a sensible heat of only 4 J/g while its temperature rises by one degree C. Therefore, the latent heat absorption during the phase change from ice into water is nearly 100 times higher than the sensible heat absorption during the heating process of water outside the phase change temperature range.
In addition to ice (water), more than 500 natural and synthetic phase change materials are known. These materials differ from one another in their phase change temperature ranges and their latent heat storage capacities.
Currently, only crystalline alkyl hydrocarbon phase change materials having different chain lengths are used in textile applications and more specifically in garment applications. Characteristics of these phase change materials are summarized in Table 1.
TABLE 1Crystalline alkyl hydrocarbonsCrystal-LatentCrystallineMeltinglizationheat storagealkyltemperature,temperature,capacity,hydrocarbonsFormula° C.° C.J/gHeneicosaneC21H4440.535.9213EicosaneC20H4236.130.6247NonadecaneC19H4032.126.4222OctadecaneC18H3828.225.4244HeptadecaneC17H3621.716.5213
The crystalline alkyl hydrocarbons are either used in technical grades with a purity of approximately 95% or they are blended with one another in order to cover specific phase change temperature ranges. The crystalline alkyl hydrocarbons are nontoxic, non-corrosive, and non-hygroscopic. The thermal behavior of these phase change materials remains stable under permanent use. Crystalline alkyl hydrocarbons are byproducts of petroleum refining and, therefore, inexpensive.
Salt hydrates are alloys of inorganic salts and water. The most attractive properties of salt hydrates are the comparatively high latent heat storage capacities, the high thermal conductivities and the small volume change during melting. Salt hydrates often show an incongruent melting behaviour as a result of a lack in reversible melting and freezing making them unsuitable for permanent use. Salt hydrates with reversible melting and freezing characteristics are summarized in Table 2.
TABLE 2Salt hydratesMeltingtemperature,Latent heat storageSalt hydrates° C.capacity, J/gCalcium cloride hexahydrate29.4170Lithium nitrate trihydrate29.9296Sodium hydrogen phosphate36.0280dodecahydrate
In the present applications of the phase change material technology in textiles, the crystalline alkyl hydrocarbon are microencapsulated, i.e., contained in small micro-spheres with diameters between 1 micron and 30 microns. These microcapsules with enclosed phase change material are applied to a textile matrix by incorporating them into acrylic fibers and polyurethane foams or by embedding them into a coating compound and coating them onto textile surfaces.
U.S. Pat. No. 4,756,958 reports a fiber with integral micro-spheres filled with phase change material which has enhanced thermal properties at predetermined temperatures.
U.S. Pat. No. 5,366,801 describes a coating where micro-spheres filled with phase change material are incorporated into a coating compound which is then topically applied to fabric in order to enhance the thermal characteristics thereof.
U.S. Pat. No. 5,637,389 reports an insulating foam with improved thermal performance, wherein micro-spheres filled with phase change material are embedded.
The micro-encapsulation process of crystalline alkyl hydrocarbon phase change materials is a very time-consuming and complicated chemical process running over several stages making the microcapsules with enclosed phase change material very expensive.
There are several thermal effects which can be obtained by a phase change material application in a certain product, such as:                A cooling effect, caused by heat absorption of the phase change material.        A heating effect, caused by heat emission of the phase change material.        A thermo-regulating effect, resulting from either heat absorption or heat emission of the phase change material.        
The efficiency of each of these effects is determined by the latent heat storage capacity of the phase change material, the phase change temperature range and the structure of the carrier system.
The total latent heat storage capacity of the phase change material in a product depends on the phase change material's specific latent heat storage capacity and its quantity. In order to obtain a successful phase change material application, the phase change temperature range and the application temperature range need to correspond.
Currently, membrane materials used for fabric structures are made of polyvinyl chloride (PVC)-coated woven polyester, poly tetra fluorine ethylene (PTFE)-coated woven fiberglass and silicone-coated woven fiberglass. The woven polyester or fiberglass fabrics provide the basic structure of the material. The mechanical properties (tensile strength, elongation and dimensional stability) of the membrane material are determined by the mechanical properties provided by the fabric construction. The PVC, PTFE or silicone coating is provided to one or both sides of the fabric in order to protect the fabrics against the infiltration of moisture, chemicals and micro-organism which could destroy the fabric construction and reduce its tensile strength. Furthermore, the coating of the fabric surface fixes the unstable fabric geometry and protects the fibers from the sun's damaging ultra violet rays.
All of the membrane materials used for fabric structures are lightweight and thin. Therefore, their thermal resistance is very low compared to other building materials. In order to improve the thermal performance of membrane roofs, they are arranged in several layers with air layers in between. The visible light transmission of PVC coated polyester fabrics and PTFE coated fiberglass fabrics does not exceed 20% and is, therefore, very low.
Membrane materials used in fabric structures have a limited service life of about 25 years due to the aging of synthetic materials. The aging of synthetic materials is caused by the ultra violet portion of the solar radiation. As a result of this radiation, chemical and physical-chemical reactions take place which lead to changes in the molecular structure of the membrane's coating. The aging process is accelerated by high temperatures to which the membrane material can be brought up during the afternoon hours and substantial temperature fluctuation during the day.